Curtain coating is a very efficient method of manufacturing precision coatings of multiple layers. There are a number of patents that describe the method, such as U.S. Pat. No. 3,508,947. In the curtain coating process, particularly when multiple layers are involved, it is critical that the thickness of each layer remain uniform over the width of the web. There are several problems that can cause layer nonuniformities before the liquid layers meet the web. For example, problems can occur at the inlets and outlets of the liquid layers flowing over the slide. Here, the liquid layers emerge from the slots onto the slide and the film thickness becomes variable in the direction transverse to the average flow. Another problem area is the precise shape of the lip from which the liquid film plunges into free fall as a curtain. Here, a contact line between the slide (solid), the liquid and the air (gas) is formed and the shape of the lip is a critical part in ensuring that the contact line is always straight. An example of a patent which describes these problems is U.S. Pat. No. 4,109,611.
Another problem area is found along each of the two sides of the hopper's slide, where the liquid's free surface meets a solid surface. These solid surfaces at the edge of the coating liquid layers must be carefully designed so that the free surface i.e., the upper interface of the top layer, remains approximately flat right up to the solid edge. This is described in U.S. Pat. No. 3,289,632. One approach to achieve this result is to design the solid surface as an edge pad with its height substantially the same as the liquid film thickness. The free surface is then pinned to the corner of the edge pad and maintains a substantially flat free surface over the width of the slide. The same approach can be used on a vertical flat section which ends at the lip where the coating liquids plunge into free fall. A related issue is that edge pads produce better results if they are continuous. Further information on apparatus for continuous edging can be found in WO94/08272 . Discontinuities in the height of the edge pads introduce flow disturbances that can produce layer thickness non-uniformities. It is inferred from this that besides being of substantially the same height as the liquid film, it is best that any difference in height between the edge pad and the adjacent liquid film vary slowly.
The problem that the present invention addresses is improving the uniformity of the liquid film's edge on the slide hopper by the matching of edge pads and the liquid film's free surface along the transition section of the slide hopper. In the transition section, the film thickness changes from a thicker film on a moderately inclined main slide to a thinner film on a much steeper slide which leads to the hopper lip. The moderate and steep inclined slide sections are inclined with respect to the horizontal between 5.degree. and 30.degree., and 60.degree. to 120.degree., respectively. The way this transition occurs can be quite complex, depending on the physical conditions of the film (flow rate, viscosity, surface tension, density) and the geometry of the transition section (length scale and shape of the section). For instance, standing waves are usually present near both ends of the transition section. These waves vary in amplitude and length scale making it very difficult to design an edge pad for the transition section that handles the many flow conditions encountered. The object of this invention is, therefore, to create a transition section on which edge pads can be mounted that will handle flow variations, as long as the initial film thickness is substantially the same. Furthermore, the film thickness along the transition section should be predictable and the length of the transition section should be sufficiently large so that edge pads can be reliably manufactured and mounted on the hopper slide surface. However, the transition section size should be reasonably short so as to reduce the possibility of creating non-uniformities in the coating due to other disturbances such as interfacial waves in multilayer film or air currents impinging on the slide surface.
Prior art transition sections are disclosed in U.S. Pat. Nos. 4,109,611 and 4,510,882. These patents describe transition sections that are sections of circular cylinders with large radii of curvature, between 20 and 50 mm. According to these patents, transition sections with large radii of curvature improve curtain stability and reduce disturbances along the lip. However, these patents fail to recognize the problem of non-uniformities along the sides of the transition section. Another prior art patent, UK 1,574,241 proposes a transition section of variable slope, preferably of a parabolic shape, of about 5 to 7 inches long for coaters with a short curtain drop i.e., less than 1 inch. The authors of that patent maintain that the variable slope accelerates the flow down the slide by such a degree that it enables the coater to enjoy the advantages of curtain coaters with substantially higher drops.
These prior art devices achieve slow changes in film thickness along their transition section. Thus, along the transition it is quite possible that the thickness of the film meets the goals of smooth and predictable film thickness variation and independence of the thickness from other flow conditions. However these sections are longer than desirable, and if they are shortened to their lowest recommended size, 20 mm, it can be inferred from the results given in the examples of the present invention that standing waves will appear near both ends of the transition section.